1. Field of the Invention
The present invention relates to a retaining structure for a honeycomb structure, and to a marine propulsion device.
2. Description of the Related Art
Installation of a honeycomb structure in the exhaust device of an engine is known in the prior art. For example, a catalytic converter disclosed in Japanese Laid-open Patent Application No. 2007-100608 has a catalyst support, a retainer mat, and a housing. The catalyst support is composed of a honeycomb structure. The housing is formed of a thin sheet material, and directly retains the honeycomb structure which is wrapped in the retainer mat. The housing is welded to an exhaust manifold, and constitutes an exhaust passage.
Because the honeycomb structure is disposed midway along the exhaust passage, it is exposed to high-temperature exhaust gas. Consequently, the housing which retains the honeycomb structure reaches a high temperature as a result of being directly affected by heat emanating from the honeycomb structure. A housing designed to directly retain the honeycomb structure as with the catalytic converter of Japanese Laid-open Patent Application No. 2007-100608 is typically formed of a heat-resistant material such as stainless steel or the like, in order to avoid erosion at high temperatures.
According to an exhaust device disclosed in Japanese Laid-open Patent Application No. 10-317947, a coolant passage is disposed at the outer perimeter of the housing. However, in this exhaust device, a portion of the housing is not covered by the coolant passage. Consequently, with the housing of Japanese Laid-open Patent Application No. 10-317947, as with the housing of Japanese Laid-open Patent Application No. 2007-100608, it has been considered that the component may be formed from a heat-resistant material such as stainless steel or the like.
However, because stainless steel is heavy, stainless steel housings are not appropriate for engines that are required to be lighter in weight. Also, because the housing itself becomes extremely hot, there is a possibility of surrounding constituent components being thermally affected thereby.
For example, in an outboard motor, a lighter weight of the engine is required due to the fact that the weight of the engine significantly affects the performance of the boat. Also, in outboard motors and personal watercraft (PWC), the exhaust system constituent components, electrical components, and so on are laid out within a limited space inside a watertight engine case. Because of this, if the honeycomb structure is disposed close to the engine, there is a possibility of heat from the housing containing the honeycomb structure adversely affecting the other constituent components.